1. Field of the Invention
The invention relates to a process of producing silicon bodies for solar cells and somewhat more particularly to an improved process for producing plate-, tape-, or film-shaped Si crystal bodies having crystalline pillar-like structures therein, which are equivalent to crystalline columnar structures, and which bodies are useful for further processing into large-surface solar cells.
2. Prior Art
The process disclosed in the above-referenced Grabmaier et al application relates to manufacture of plate-shaped or tape-shaped Si crystal bodies having crystalline pillar-like structures therein, equivalent to crystalline columnar structures, and which bodies are useful, preferably for processing into solar cells.
As is known from prior art publications, a number of methods for producing Si crystal bodies for solar cells are known. However, these methods are typically too costly, in particular because they require sawing of a Si body produced in accordance with conventional crystal growth techniques, which are known per se, to an appropriate crystal disc required for solar cells.
The earlier referenced Grabmaier et al application discloses a process for producing Si crystal bodies which, during the manufacture thereof, are already in the form of plates, tapes or films and have crystalline pillar-like structures therein, equivalent to crystalline columnar structures, so that such bodies can readily be further processed to completion into finished large-surface solar cells. The Grabmaier et al process occurs without melting of the base or starting materials forming the Si bodies. The starting materials comprise a Si powder having an average particle diameter in the range of less than about 1 .mu.m, which is admixed with a suitable binder, such as aqueous polyvinyl alcohol, and optional additives, to form a slurry. This slurry is extruded onto an inert support member as a relatively thin film or the like via an extrusion means, such as a doctor-blading tool, dried in air so as to form a self-supporting layer so that the support can be removed. The dried slurry layer is then placed on a temperature-resistant inert base and sintered in a protective gas atmosphere at a sintering temperature below about 1430.degree. C. in such a manner that a layer or film is generated having monocrystalline silicon particles therein with an average particle diameter corresponding to the thickness of the sintered layer. In certain embodiments of this Grabmaier et al process, sintering aids, such as germanium, can be added to the silicon powder in an amount up to a maximum of about 5% by weight. Further, during sintering, the heat distribution within the sintering furnace is preferably so adjusted that a temperature gradient is attained in the thickness direction of the film or layer being sintered. The support base for the sintering step is preferably composed of quartz glass and is provided with a periodic spacing of crystallization seed centers which promote formation of the desired crystalline pillar structures. Such seed centers can be peak-shaped elevations uniformly spaced at intervals matched to the desired crystalline columnar or pillar structures of the silicon material. Select dopants, for example, arsenic and/or boron, along with any sintering aids, in the form of an arsenic-containing or boron-containing germanium alloy, can be added during slurry formation so as to be substantially uniformly distributed throughout the formed Si crystalline body. Such doped Si bodies are especially useful for fabrication into solar cells.
The manufacturing process disclosed in the above referenced Grabmaier et al application is particularly economical when work is carried out in a continuous manner whereby during extrusion of the silicon layer or film with a stationary extrusion shoe (or doctor-blading tool), the underlying support base is moved in a given direction at a preselected speed. Prior to actual sintering, the extruded tape-shaped silicon layer or film can be divided into individual plates or tiles corresponding to desired dimensions of solar cells to be produced therefrom. In this manner, not only is sawing of elongated crystalline Si bodies into discs or plates eliminated, but also the division of such body into Si plates of a select size can occur prior to sintering.
In the foregoing process, sintering of the Si slurry layer into Si crystalline bodies occurs at temperatures below the melting point of Si (1430.degree. C.). Under these conditions and with appropriate heat distribution within the sintering furnace, larger crystals grow at the expense of smaller crystals at an orientation essentially parallel to the thickness dimension of the Si layer. The same effect can also be attained with the aid of a support member composed, for example, of quartz glass, and having the earlier described periodicity of singularities, particularly peak-shaped elevations, which together with a suitably matched heat distribution, initiate crystal growth on the contact surface between the support member and the Si layer positioned thereon and favors the desired crystalline pillar or columnar structures in the thickness dimension of the Si layer.